单项选择题 若=A (A为常数)，则下列结论正确的是 A. 不一定存在； B. ； C. 存在，且=A D. 在x=a 处不一定连续．

单项选择题 28.患者男性39岁，阵发性右下腹部绞痛，疼痛剧烈难忍，镜下血尿，DR检查，右侧输尿管下段有密度比较高的0.8×1.0cm大小阴影。诊断为右侧输尿管结石。输尿管有几处狭窄(    )A. 2个B. 3个C. 4个D. 1个E. 以上均不是

单项选择题 It seems oil ______ from this pipe for some time. We'll have to take the machine apart to put it right.    A. had leaked    B. is leaking         C. leaked         D. has been leaking